The machining of workpieces is often complex, and can encompass a plurality of chip-generating and non-chip-generating working steps on various processing machines in order to produce the finished component from a blank. In all production sequences, it is essential for the workpiece to occupy a defined location on the particular machine tool for each working step.
For this purpose, it is usual in practice first to clamp the workpiece that is to be machined onto a carrier plate, a clamping plate, or the like, and then to clamp the entire unit in place on the particular machine tool by way of several draw-in bolts that are mounted on the back side of the carrier plate and embodied as centering elements, by inserting the draw-in bolts into corresponding quick-release clamping units that are provided on the machine table. In production, the location of the quick-release clamping devices can therefore be regarded as a fixed machine parameter, so that it is necessary only to position the respective workpiece exactly on the carrier plate. The unit made up of the workpiece and carrier plate can then be actually clamped and reclamped on the machines very quickly and without difficulty, with no need for further positioning.
In conventional clamping devices of the kind cited initially, as known for example from DE 200 03 729 U1, the draw-in bolt is inserted into a receptacle of the clamping device which is embodied so that the inserted draw-in bolt is centered by its contact with the inner wall of the receptacle. Retention of the draw-in bolt in the receptacle is accomplished by means of gripping elements that are embodied as resilient clamping tongues.
From EP 1 264 657 A1 it is furthermore known to seal a receptacle of a clamping device in such a way that dirt particles cannot penetrate into it even when the draw-in bolt is removed. For that purpose, a plate-shaped closure cap is inserted into the receptacle and can be moved by a closing piston between two end positions. In its extended end position, in which its end surface facing toward the outer side of the clamping device terminates flush with the surface of the clamping device, the receptacle is protected from contamination. In the retracted end position, the draw-in bolt can be inserted into the receptacle.
With the known clamping devices, it is regarded as disadvantageous to some extent that the draw-in bolt must be inserted in accurately fitted fashion into the receptacle.